Population dynamics and infection prevalence of grasshopper (Orthoptera: Acrididae) after application of Paranosema locustae (Microsporidia).

The substantial harm to ecosystems resulted from applying chemical pesticides has attracted the interest to biopesticides for controlling grasshoppers. Paranosema locustae was applied for controlling grasshoppers at Qilian rangeland in 2000. Population of the most abundant grasshopper, Myrmeleotettix palpalis, declined from 13.04/m(2) in the year of treatment to 6.96/m(2) and 1.14/m(2) by the first and second years after treatment. Population of this species fluctuated regularly in years 2-5 post-treatment with densities varying from 1.14 to 3.52/m(2), while it increased after the fifth year and peaked (9.09/m(2)) by the seventh year. However, there were large increases in grasshopper populations in the treated area with chemical insecticide to 35.3/m(2) after the fifth year leading to re-treatment with a chemical insecticide. In the year of treatment, 51.64-65.39% of surviving grasshoppers of different species were found infected. Disease prevalence was 66.11-68.36% after the first year for these primary grasshopper species, but gradually decreased by approximately 20-30% by the 6th year where it remained for the rest of the period, thus indicating that the pathogen remained in grasshopper populations for successive years. Infection prevalence of grasshoppers and natural enemies may be vital in keeping densities below economic injury level (EIL) for more than nine years after treatment with P. locustae.